Nikolai N. Faleev

High quality GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.

Dependence of the stress–temperature coefficient on dislocation density in epitaxial GaN grown on α-Al2O3 and 6H–SiC substrates

We report measurements of stress in GaN epitaxial layers grown on 6H–SiC and α-Al2O3 substrates. Biaxial stresses span +1.0 GPa (tensile) to −1.2 GPa (compressive). Stress determined from curvature measurements, obtained using phase-shift interferometry (PSI) microscopy, compare well with measurements using accepted techniques of x-ray diffraction (XRD) and Raman spectroscopy. Correlation between XRD and Raman measurements of the E22 phonon gives a Raman-stress factor of −3.4±0.3 cm−1/GPa. We apply PSI microscopy for temperature dependent stress measurements of the GaN films. Variations found in the stress–temperature coefficient correlate well with threading dislocation densities. We develop a phenomenological model which describes the thermal stress of the epitaxial GaN as a superposition of that for ideal GaN and the free volume existing in the layers due to the threading dislocations. The model describes well the observed dependence.

High-quality AlN grown on Si(111) by gas-source molecular-beam epitaxy with ammonia

Hexagonal AlN layers were grown on Si(111) by gas-source molecular-beam epitaxy with ammonia. The transition between the (7×7) and (1×1) silicon surface reconstructions, at 1100 K, was used for in situ calibration of the substrate temperature. The initial deposition of Al, at 1130–1190 K, produced an effective nucleation layer for the growth of AlN. The Al layer also reduced islands of SiNx that might be formed due to background NH3 on the silicon surface prior to the onset of epitaxial growth. The transition to two-dimensional growth mode, under optimum conditions, was obtained after the initial AlN thickness of ∼7 nm.

Selective growth of high quality GaN on Si(111) substrates

We demonstrate selective growth of high-quality GaN by gas-source molecular beam epitaxy on Si(111) wafers patterned with SiO2. GaN was grown on wafers having two different buffer layers. The first buffer layer contains two AlGaN/GaN superlattices, separated by GaN spacer, grown on AlN, with a total thickness of 400 nm. The second is a thin AlN (1.5 nm) buffer layer. X-ray diffraction confirms (0001) growth orientation, smooth interfaces, and coherence lengths comparable to the layer thickness in both samples. In the case of the thin AlN buffer layer, the tensile stress measured by the E2 Raman line shift is attributed to the mismatch in the thermal expansion coefficients of GaN and Si. However, when the AlGaN/GaN superlattice buffer layer is grown first, a reduced stress is measured. High carrier concentrations (≈1018 cm−3) are seen in the GaN grown on the thin AlN buffer layer, which we attribute to the incorporation of silicon from the substrate during the growth process. The superlattice buffer layer ...

Growth of AlGaN on Si(111) by gas source molecular beam epitaxy

Gas source molecular beam epitaxy with ammonia was used to grow AlxGa1−xN on Si(111). Three types of buffer layers, containing AlN, AlGaN/AlN, and AlGaN/GaN short period superlattices, were used and their effectiveness evaluated by x-ray diffraction. We determined that a combination of AlN buffer layer, prepared under the two-dimensional growth mode, with a short period superlattice of AlGaN/GaN results in the highest quality AlGaN. Under optimized growth conditions, x-ray diffraction coherence length almost equal to the layer thickness was obtained for low Al content layers. The normalized coherence length was reduced to ∼0.4 for x=0.66 and it increased again to ∼0.75 in AlN. From room temperature band edge cathodoluminescence of AlGaN grown on Si(111) we determined the alloy bowing coefficient of b=1.5 eV, in good agreement with previous results obtained by absorption measurements.

Single phase ZnSnAs2 grown by molecular beam epitaxy

Epitaxial layers of ZnSnAs2 were grown by molecular beam epitaxy on Si and InP substrates. Growth conditions were investigated by varying the substrate temperature and the Sn, As, and Zn fluxes. The best morphology and stoichiometry was obtained at Ts=300–320 °C and the flux ratio of PAs4/PZn∼1.5–4. The samples were evaluated by secondary neutral mass spectroscopy, high resolution x-ray diffraction, Raman spectroscopy, and atomic force microscopy. Single phase layers of ZnSnAs2 grown on InP(001) substrates show lattice mismatch of −3.4×10−4.

Gas source molecular beam epitaxy of GaN with hydrazine on spinel substrates

Growth of high quality wurtzite-structure GaN layers on (111) MgAl2O4 by gas source molecular beam epitaxy is described. Hydrazine was used as a source of active nitrogen. In situ reflection high energy electron diffraction was used to monitor the growth mode. Two-dimensional growth was obtained at temperatures above 750 °C on multi-step GaN buffer layers. The resulting GaN films show excellent luminescence properties.

High-resolution x-ray study of thin GaN film on SiC

The x-ray standing wave method (XSW) and high-resolution x-ray diffraction were used to study the structural perfection and polarity of GaN epitaxial thin film grown by hydride vapor phase epitaxy on the Si-face SiC substrate. The x-ray standing wave was generated inside the 300 nm thin film under the condition of Bragg diffraction from the film. Excellent crystalline quality of the GaN film was revealed by both x-ray techniques. The XSW analysis of the angular dependencies of the Ga–K fluorescence yield measured while scanning through the GaN(0002) diffraction peak unambiguously showed the Ga polarity of the film. Correlation between the mosaic structure and the static Debye–Waller factor of the GaN lattice was also studied.

Luminescence of GaN/GaAs(111)B grown by molecular beam epitaxy with hydrazine

Hexagonal GaN layers were grown on (111)B GaAs substrates by gas-source molecular beam epitaxy using hydrazine as a source of nitrogen. A smooth and abrupt AlN/GaAs interface was prepared by nitridation of an AlAs buffer layer grown on clean GaAs(111)B surface. This buffer layer is stable at growth temperatures above 700 °C. The AlN layer prepared on this buffer exhibits two-dimensional growth. The subsequent GaN and GaInN layers show a quasi-two-dimensional growth. The photo-and cathodoluminescence spectra of these samples show a narrow (∼160 meV) band-edge emission and the absence of the “yellow” defect band. Further narrowing (∼20 meV) of the edge emission in unintentionally In-doped GaN is ascribed to the presence of confined domains of Ga1−xInxN.

AlGaAs photovoltaic arrays spectrally matched to photoluminescent fibers for UGS application

This paper reports progress in the development of a miniature photovoltaic (PV) arrays consisting of monolithically series connected AlGaAs/GaAs PV cells used in combination with polymeric photoluminescent fibers to recharge batteries of unattended ground sensors (UGS). Outdoor tests of the arrays showed feasibility of this approach. Optimization of the fibers design (material used, diameter, coupling, etc.) is discussed. Better optical matching of the fibers and PV cells was achieved through replacing of GaAs photoactive layers by AlGaAs ones having a higher bandgap.